Khmer Economy
Angkor was located in a very fertile area just North of the Great Lake, Tonle Sap. This meant that they could grow almost anything and their agriculture was their main source of economical profit. Angkor traded with neighboring tribes and settlements and made a lot of profit from fish, water supplies and backflows from the Mekong River.

<-- The Tonle Sap Lake

Agriculture
The Khmers used a very effective technique to grow their rice. They called it "floating rice", which they could grow very fast in only 3 meters of water. This meant that they could have at least 3 harvests a year while normal civilizations usually had one. This allowed their agricultural economy to boom tremendously and allowed them to make a lot of profit, with additional surplus food to feed all of the people in the empire.